In the field of flexographic printing ink samples may be obtained by drawing ink over a substrate using a hand ink proofer or by more sophisticated proofing methods. In hand proofing ink is applied to the substrate by manually rolling the hand proofer across the substrate. Manual ink proofer tools are utilized for proofing ink colors in an effort to accurately predict the results to be obtained by running a selected ink specimen in a printing press. A computer microscope or other instrument is then used to examine the ink smear on the substrate. The computer then indicates to the technician various color components to be added to the ink in order to achieve the desired ink coloration.
In a flexographic printing operation, resilient plates are utilized for delivering the ink to the substrate. Substrates generally include the stock or paper to be printed but may also include plastic and many other materials.
The shade of a color on a flexographic printing press is dependent on the thickness of the ink film applied to the substrate or stock. The ink film thickness is determined by the speed of the press, the pressure applied between the printing plate and paper (i.e., impression), and the pressure between the rollers on the printing unit.
U.S. Pat. No. 6,814,001 describes an ink proofer designed to overcome the problems associated with conventional manual proofer tools by generating consistent and reliable ink draws using a hand-held proofer tool retained in a movable mounting assembly. A variable pressure system is coupled to the mounting assembly to move the proofer tool into a contact position with a cylindrical drum. The transfer roller of the proofer tool then transfers ink to a substrate inserted between the drum and the transfer roller of the proofer tool when a drive motor for the drum is engaged. U.S. Pat. No. 6,814,001 is hereby incorporated by reference.
Printing presses generally use an anilox roll to meter ink and a cylinder bearing an engraved plate to transfer the ink from the anilox roll and to deposit it onto the substrate as a printed image. The substrate commonly includes paper but may also include many other materials such as plastic bags or any other material onto which printing may be applied.
The engraved plate may be made to include both solid and/or dot patterns depending upon image requirements. For a single color image, typically a plate with a solid or smooth surface may be used. For a multi-color image where more than one color is required a dot pattern is generally used. The superimposition of multiple dot patterns onto a substrate is used to print multi-color images. Typically each dot pattern is printed with a primary color onto the substrate. By putting the substrates through multiple passes in the press, any shade or color may be created by the combination of primary colors.
To obtain the desired colors in multi-color materials however, each primary color must print correctly and be of the correct density. Therefore, when adjusting inks for color, it is the primary color in each dot pattern that must be controlled.
Current proofing processes only use an anilox in a transfer roll to lay down ink. This process creates a smear of ink that proofs its color and density. The transfer roll duplicates the volume of the ink in the anilox and color, but does not duplicate the dot percentage pattern found in an offset plate. The dot percentage pattern is based on the proportion of the substrate that is covered with ink. Small dots result in a smaller percentage of coverage than large dots.
Printing plates can be and often are tested on the printing press but the expense of doing so is high. Modern printing presses are expensive. Any time that is used to test on the press is non productive time and cannot be used for profitable production. A printing press requires considerable time for setup and cleanup in addition to the time that is used in a test run. In addition, modern printing presses operate at high speed and can consume large quantities of ink and substrate quickly adding to the expense of testing.
Thus, there is still room for improvement in the preparation of proofing printouts in order to provide the best results in a printing press. While current proofing techniques are helpful in preparing for production printing press runs they are not adequate to predict the performance of the printing press.
A standard flexographic printing press has four main components:
1. A metering roll. This roll rotates in an ink well, wherein ink adheres to its surface. The ink well is necessarily located below the metering or inking roll because of gravity. As such the metering roll is located below and in contact with the anilox roll. As the metering roll rotates, it contacts the anilox roll and squeezes the ink into the anilox cells.
2. The anilox rotates, laden with ink, to the doctor blade.
3. The doctor blade is located near the metering roll, and presses against the anilox, and also is positioned with the edge “leading” (or cutting) into the anilox. This position shears the excess ink from the anilox and, usually, returns it to the ink well via gravity.
4. The photopolymer plate or other printing plate then receives the ink from the anilox and transfers it to the substrate.
A conventional hand proofer has three components:
1. An anilox which transfers metered ink from the doctor blade to the transfer roll or photopolymer plate.
2. A doctor blade located near the top of the anilox and positioned with its edge trailing (creating wiping action). This wiping action is necessary because ink needs to be forced into the anilox before metering it. So a trailing edge anilox services two purposes, forcing ink into anilox and metering it level to the anilox surface. These two actions compromise each others' ability to perform. The wiping action of the trailing blade tends to lift the doctor blade, allowing non-metered ink to pass. This non-metered amount of ink reduces accuracy of ink application and reduces the quality and consistency of the resulting proof.
3. An ink transfer roll that receives ink from the anilox and applies an ink sample to the substrate.